This invention relates to wastewater treatment, and in particular to a two-stage biological oxidation treatment comprising: (a) decomposing the major quantity of organic pollutants in the first stage, and (b) decomposing residual amounts of pollutants in the second stage wherein nitrogen compounds are also oxidized in the presence of nitrifying bacteria.
In such a process, wastewater is conventionally initially mixed, in the first stage, with activated sludge with the simultaneous introduction of molecular oxygen in the form of air or a gas having a higher oxygen concentration. By virtue of the activity of the aerobic microorganisms contained in the activated sludge, the organic pollutants of the wastewater are decomposed or degraded, the specific mechanism being conversion, in part, into bacterial substance and, in part, into CO.sub.2 and water, the overall effect in the first stage being extensive oxidation of the carbon compounds. After passing resultant wastewater-activated sludge mixture from the first stage into the second stage, residual degradation takes place in the latter stage, along with nitrification of the wastewater, assuming there is a sufficient oxygen supply, a sufficient water temperature, and a sufficiently low sludge load. In this nitrification process, ammonium nitrogen (NH.sub.4.sup.+ -N) is practically completely oxidized to nitrate by the autotrophic bacteria growing in the sludge. To supply oxygen to the bacteria, it is also old to introduce into the aeration tank of the second stage as well, molecular oxygen in the form of air or a gas containing even a higher percent by volume of oxygen.
Since in such a method the primary amount of organic pollutants is degraded in the first stage, only a minor BOD load exists in the second stage, and for this reason, due to lack of nutrients and suspended solids, a sludge is formed in the second stage which settles very slowly and usually requires large post clarification tanks with sludge-removing devices, particularly in the case of the existence of floating sludge. However, even with the provision of large post clarification tanks, under conditions of a high hydraulic load and sludge drifting (which occurs in any treatment plant due to denitrification processes), a large proportion of the sludge particles passes over into the drainage canal from the post clarification tank, together with the slowly growing nitrifying bacteria. Aside from the fact that this spillover containing suspended matter impairs the overall quality of the resultant treated wastewater discharged from the plant, there occurs a significant disadvantage insofar as a part of the nitrifying bacteria is thus no longer available for the nitrification process.